Molecular cloning of a ligand for the EPH-related receptor protein-tyrosine kinase Htk

Polydom/SVEP1 binds to Tie1 and promotes migration of lymphatic endothelial cells

Polydom is an extracellular matrix protein involved in lymphatic vessel development. Polydom-deficient mice die immediately after birth due to defects in lymphatic vessel remodeling, but the mechanism involved is poorly understood. Here, we report that Polydom directly binds to Tie1, an orphan receptor in the Angiopoietin-Tie axis, and facilitates migration of lymphatic endothelial cells (LECs) in a Tie1-dependent manner. Polydom-induced LEC migration is diminished by PI3K inhibitors but not by an ERK inhibitor, suggesting that the PI3K/Akt signaling pathway is involved in Polydom-induced LEC migration. In line with this possibility, Akt phosphorylation in LECs is enhanced by Polydom although no significant Tie1 phosphorylation is induced by Polydom. LECs also exhibited nuclear exclusion of Foxo1, a signaling event downstream of Akt activation, which was impaired in Polydom-deficient mice. These findings indicate that Polydom is a physiological ligand for Tie1 and participates in lymphatic vessel development through activation of the PI3K/Akt pathway.

EGFR in enterocytes & endothelium and HIF1α in enterocytes are dispensable for massive small bowel resection induced angiogenesis

Background

Short bowel syndrome (SBS) results from significant loss of small intestinal length. In response to this loss, adaptation occurs, with Epidermal Growth Factor Receptor (EGFR) being a key driver. Besides enhanced enterocyte proliferation, we have revealed that adaptation is associated with angiogenesis. Further, we have found that small bowel resection (SBR) is associated with diminished oxygen delivery and elevated levels of hypoxia-inducible factor 1-alpha (HIF1α).

Methods

We ablated EGFR in the epithelium and endothelium as well as HIF1α in the epithelium, ostensibly the most hypoxic element. Using these mice, we determined the effects of these genetic manipulations on intestinal blood flow after SBR using photoacoustic microscopy (PAM), intestinal adaptation and angiogenic responses. Then, given that endothelial cells require a stromal support cell for efficient vascularization, we ablated EGFR expression in intestinal subepithelial myofibroblasts (ISEMFs) to determine its effects on angiogenesis in a microfluidic model of human small intestine.

Results

Despite immediate increased demand in oxygen extraction fraction measured by PAM in all mouse lines, were no differences in enterocyte and endothelial cell EGFR knockouts or enterocyte HIF1α knockouts by POD3. Submucosal capillary density was also unchanged by POD7 in all mouse lines. Additionally, EGFR silencing in ISEMFs did not impact vascular network development in a microfluidic device of human small intestine.

Conclusions

Overall, despite the importance of EGFR in facilitating intestinal adaptation after SBR, it had no impact on angiogenesis in three cell types–enterocytes, endothelial cells, and ISEMFs. Epithelial ablation of HIF1α also had no impact on angiogenesis in the setting of SBS.

Genome-Wide Association Study Using Individual Single-Nucleotide Polymorphisms and Haplotypes for Erythrocyte Traits in Alpine Merino Sheep

Adaptation to high-altitude hypoxia is essential for domestic animals, such as yak, Tibetan chicken, and Tibetan sheep, living on high plateaus, as it ensures efficient oxygen absorption and utilization. Red blood cells are the primary medium for transporting oxygen in the blood. However, little is known about the genetic mechanism of erythrocyte traits. Genome-wide association studies (GWASs) based on single markers or haplotypes have identified potential mechanisms for genetic variation and quantitative traits. To identify loci associated with erythrocyte traits, we performed a GWAS based on the method of the single marker and haplotype in 498 Alpine Merino sheep for six erythrocyte traits: red blood cell count (RBC), hemoglobin (HGB), hematocrit (HCT), mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC), and RBC volume distribution width coefficient of variation (RWD_CV). Forty-two significant single-nucleotide polymorphisms (SNPs) associated with the six erythrocyte traits were detected by means of a single-marker GWAS, and 34 significant haplotypes associated with five erythrocyte traits were detected by means of haplotype analysis. We identified six genes (DHCR24, SPATA9, FLI1, PLCB1, EFNB2, and SH2B3) as potential genes of interest via gene function annotations, location, and expression variation. In particular, FLI1 and PLCB1 were associated with hematopoiesis and erythropoiesis, respectively. These results provide a theoretical basis for analyzing erythrocyte traits and high-altitude hypoxia adaptation in Alpine Merino sheep and will be a useful reference for future studies of plateau-dwelling livestock.

Arteriogenesis of the Spinal Cord-The Network Challenge

Spinal cord ischemia (SCI) is a clinical complication following aortic repair that significantly impairs the quality and expectancy of life. Despite some strategies, like cerebrospinal fluid drainage, the occurrence of neurological symptoms, such as paraplegia and paraparesis, remains unpredictable. Beside the major blood supply through conduit arteries, a huge collateral network protects the central nervous system from ischemia—the paraspinous and the intraspinal compartment. The intraspinal arcades maintain perfusion pressure following a sudden inflow interruption, whereas the paraspinal system first needs to undergo arteriogenesis to ensure sufficient blood supply after an acute ischemic insult. The so-called steal phenomenon can even worsen the postoperative situation by causing the hypoperfusion of the spine when, shortly after thoracoabdominal aortic aneurysm (TAAA) surgery, muscles connected with the network divert blood and cause additional stress. Vessels are a conglomeration of different cell types involved in adapting to stress, like endothelial cells, smooth muscle cells, and pericytes. This adaption to stress is subdivided in three phases—initiation, growth, and the maturation phase. In fields of endovascular aortic aneurysm repair, pre-operative selective segmental artery occlusion may enable the development of a sufficient collateral network by stimulating collateral vessel growth, which, again, may prevent spinal cord ischemia. Among others, the major signaling pathways include the phosphoinositide 3 kinase (PI3K) pathway/the antiapoptotic kinase (AKT) pathway/the endothelial nitric oxide synthase (eNOS) pathway, the Erk1, the delta-like ligand (DII), the jagged (Jag)/NOTCH pathway, and the midkine regulatory cytokine signaling pathways.

Detailed Molecular Biochemistry for Novel Therapeutic Design Against Nipah and Hendra Virus: A Systematic Review

BACKGROUND

Nipah virus (NiV) and Hendra virus (HeV) of genus Henipavirus are the deadliest zoonotic viruses, which cause severe respiratory ailments and fatal encephalitis in humans and other susceptible animals. The fatality rate for these infections had been alarmingly high with no approved treatment available to date. Viral attachment and fusion with host cell membrane is essential for viral entry and is the most essential event of viral infection. Viral attachment is mediated by interaction of Henipavirus attachment glycoprotein (G) with the host cell receptor: Ephrin B2/B3, while viral fusion and endocytosis are mediated by the combined action of both viral glycoprotein (G) and fusion protein (F).

CONCLUSION

This review highlights the mechanism of viral attachment, fusion and also explains the basic mechanism and pathobiology of this infection in humans. The drugs and therapeutics used either experimentally or clinically against NiV and HeV infection have been documented and classified in detail. Some amino acid residues essential for the functionality of G and F proteins were also emphasized. Therapeutic designing to target and block these residues can serve as a promising approach in future drug development against NiV and HeV.

Molecular identity of arteries, veins, and lymphatics

BACKGROUND

Arteries, veins, and lymphatic vessels are distinguished by structural differences that correspond to their different functions. Each of these vessels is also defined by specific molecular markers that persist throughout adult life; these markers are some of the molecular determinants that control the differentiation of embryonic undifferentiated cells into arteries, veins, or lymphatics.

METHODS

This is a review of experimental literature.

RESULTS

The Eph-B4 receptor and its ligand, ephrin-B2, are critical molecular determinants of vessel identity, arising on endothelial cells early in embryonic development. Eph-B4 and ephrin-B2 continue to be expressed on adult vessels and mark vessel identity. However, after vascular surgery, vessel identity can change and is marked by altered Eph-B4 and ephrin-B2 expression. Vein grafts show loss of venous identity, with less Eph-B4 expression. Arteriovenous fistulas show gain of dual arterial-venous identity, with both Eph-B4 and ephrin-B2 expression, and manipulation of Eph-B4 improves arteriovenous fistula patency. Patches used to close arteries and veins exhibit context-dependent gain of identity, that is, patches in the arterial environment gain arterial identity, whereas patches in the venous environment gain venous identity; these results show the importance of the host infiltrating cells in determining vascular identity after vascular surgery.

CONCLUSIONS

Changes in the vessel's molecular identity after vascular surgery correspond to structural changes that depend on the host's postsurgical environment. Regulation of vascular identity and the underlying molecular mechanisms may allow new therapeutic approaches to improve vascular surgical procedures.

Membrane-mediated regulation of vascular identity

Vascular diseases span diverse pathology, but frequently arise from aberrant signaling attributed to specific membrane-associated molecules, particularly the Eph-ephrin family. Originally recognized as markers of embryonic vessel identity, Eph receptors and their membrane-associated ligands, ephrins, are now known to have a range of vital functions in vascular physiology. Interactions of Ephs with ephrins at cell-to-cell interfaces promote a variety of cellular responses such as repulsion, adhesion, attraction, and migration, and frequently occur during organ development, including vessel formation. Elaborate coordination of Eph- and ephrin-related signaling among different cell populations is required for proper formation of the embryonic vessel network. There is growing evidence supporting the idea that Eph and ephrin proteins also have postnatal interactions with a number of other membrane-associated signal transduction pathways, coordinating translation of environmental signals into cells. This article provides an overview of membrane-bound signaling mechanisms that define vascular identity in both the embryo and the adult, focusing on Eph- and ephrin-related signaling. We also discuss the role and clinical significance of this signaling system in normal organ development, neoplasms, and vascular pathologies.

Henipavirus receptor usage and tropism

Nipah (NiV) and Hendra (HeV) viruses are the deadliest human pathogens within the Paramyxoviridae family, which include human and animal pathogens of global biomedical importance. NiV and HeV infections cause respiratory and encephalitic illness with high mortality rates in humans. Henipaviruses (HNV) are the only Paramyxoviruses classified as biosafety level 4 (BSL4) pathogens due to their extreme pathogenicity, potential for bioterrorism, and lack of licensed vaccines and therapeutics. HNV use ephrin-B2 and ephrin-B3, highly conserved proteins, as viral entry receptors. This likely accounts for their unusually broad species tropism, and also provides opportunities to study how receptor usage, cellular tropism, and end-organ pathology relates to the pathobiology of HNV infections. The clinical and pathologic manifestations of NiV and HeV virus infections are reviewed in the chapters by Wong et al. and Geisbert et al. in this issue. Here, we will review the biology of the HNV receptors, and how receptor usage relates to HNV cell tropism in vitro and in vivo.

Evidence for a dual function of EphB4 as tumor promoter and suppressor regulated by the absence or presence of the ephrin-B2 ligand

Overexpression of the receptor tyrosine kinase EphB4 is common in epithelial cancers and linked to tumor progression by promoting angiogenesis, increasing survival and facilitating invasion and migration. However, other studies have reported loss of EphB4 suggesting a tumor suppressor function in some cancers. These opposing roles may be regulated by (i) the presence of the primary ligand ephrin-B2 that regulates pathways involved in tumor suppression or (ii) the absence of ephrin-B2 that allows EphB4 signaling via ligand-independent pathways that contribute to tumor promotion. To explore this theory, EphB4 was overexpressed in the prostate cancer cell line 22Rv1 and the mammary epithelial cell line MCF-10A. Overexpressed EphB4 localized to lipid-rich regions of the plasma membrane and confirmed to be ligand-responsive as demonstrated by increased phosphorylation of ERK1/2 and internalization. EphB4 overexpressing cells demonstrated enhanced anchorage-independent growth, migration and invasion, all characteristics associated with an aggressive phenotype, and therefore supporting the hypothesis that overexpressed EphB4 facilitates tumor promotion. Importantly, these effects were reversed in the presence of ephrin-B2 which led to a reduction in EphB4 protein levels, demonstrating that ligand-dependent signaling is tumor suppressive. Furthermore, extended ligand stimulation caused a significant decrease in proliferation that correlated with a rise in caspase-3/7 and -8 activities. Together, these results demonstrate that overexpression of EphB4 confers a transformed phenotype in the case of MCF-10A cells and an increased metastatic phenotype in the case of 22Rv1 cancer cells and that both phenotypes can be restrained by stimulation with ephrin-B2, in part by reducing EphB4 levels.

Cluster analysis and phylogenetic relationship in biomarker identification of type 2 diabetes and nephropathy

Cluster analysis of DNA microarray data that uses statistical algorithms to arrange the genes according to similarity in patterns of gene expression and the output displayed graphically is described in this article. Hierarchical clustering is a multivariate tool often used in phylogenetics, comparative genomics to relate the evolution of species. The patterns seen in microarray expression data can be interpreted as indications of the status of the genes responsible for nephropathy in peripheral blow cells of type 2 diabetes (T2DN). Out of 415 genes totally expressed in the 3 DNA chips it was concluded that only 116 genes expressed in T2DN and in that only 50 are functional genes. These 50 functional genes are responsible for diabetic nephropathy; of these 50, some of the genes which are more expressed and responsible are AGXT: Alanine-glyoxylate aminotransferase, RHOD: Ras homolog gene family, CAPN6: Calpain 6, EFNB2: Ephrin-B2, ANXA7: Annexin A7, PEG10: Paternally expressed 10, DPP4: Dipeptidyl-peptidase 4 (CD26, adenosine deaminase complexing protein 2), ENSA: Endosulfine alpha, IGFBP2: Insulin-like growth factor binding protein 2, 36kDa, CENPB: Centromere protein B, 80kDa, MLL3: Myeloid/lymphoid or mixed-lineage leukemia 3, BDNF: Brain-derived neurotrophic factor, EIF4A2: Eukaryotic translation initiation factor 4A, isoform 2, PPP2R1A: Protein phosphatase 2 (formerly 2A), regulatory subunit A, alpha isoform. Fifty genes and their nucleotide sequences are taken from NCBI and a phylogenetic tree is constructed using CLUSTAL W and the distances are closer to each other concluding that based on the sequence similarity and evolution the genes are expressed similarly. Literature survey is done for each gene in OMIM and the genes responsible for diabetic nephropathy are listed.

Ex vivo gene delivery of ephrin-B2 induces development of functional collateral vessels in a rabbit model of hind limb ischemia

OBJECTIVE

In this study, we delivered ephrin-B2 to the ischemic hind limb of rabbits using an ex vivo method of gene transfer and evaluated whether the in vivo application of ephrin-B2 contributed to the development of functional collateral vessels. Ephrin-B2 is a transmembrane ligand of several Eph receptors and bidirectional signaling between ephrin-B2 and Eph-B4 is considered to be essential in angiogenesis and the development of arteries and veins.

METHOD

The left femoral artery of male Japanese White rabbits was excised to induce limb ischemia, and a primary culture of autofibroblasts was obtained from a skin section. Nineteen days later, the gene expressing ephrin-B2 (ephrin group) or beta-galactosidase gene (control group) was adenovirally transfected to the cultured auto-fibroblasts (5 x 10(6) cells); then 48 hours later, the gene-transduced cells were injected through the left internal iliac artery of the same rabbit. At 28 days after injection, the development of collateral vessels and their function were assessed (control group, n = 12; ephrin group, n = 10).

RESULTS

The gene expressing ephrin-B2 was successfully transferred to the rabbit autofibroblasts, and ephrin-B2, expressed on the cell membrane, possessed binding ability with its receptor, Eph-B4. Calf blood pressure ratio (control group: 0.523 +/- 0.047 vs ephrin group: 0.658 +/- 0.049, P < .0001), angiographic score (0.344 +/- 0.091 vs 0.525 +/- 0.109, P = .0006), in vivo blood flow of the left internal iliac artery (rest: 11.963 +/- 2.806 vs 17.202 +/- 3.622 mL/min, P = .0014; maximum: 27.652 +/- 10.377 vs 43.400 +/- 7.108 mL/min, P = .0007), collateral conductance (32.740 +/- 7.408 vs 54.489 +/- 18.809 mL/min/100 mm Hg, P = .0097), and capillary density of the left thigh muscle (118.517 +/- 18.669 vs 167.400 +/- 31.271, P = .0002) showed significant improvement in the ephrin-B2 group compared with controls.

CONCLUSION

These findings suggest that auto-fibroblasts expressing ephrin-B2 potentially promote arteriogenesis as well as angiogenesis in the adult vasculature, resulting in the development of functional collateral vessels to an ischemic lesion.

Tissue-engineered vascular grafts demonstrate evidence of growth and development when implanted in a juvenile animal model

INTRODUCTION

The development of a living, autologous vascular graft with the ability to grow holds great promise for advancing the field of pediatric cardiothoracic surgery.

OBJECTIVE

To evaluate the growth potential of a tissue-engineered vascular graft (TEVG) in a juvenile animal model.

METHODS

Polyglycolic acid nonwoven mesh tubes (3-cm length, 1.3-cm id; Concordia Fibers) coated with a 10% copolymer solution of 50:50 L-lactide and epsilon-caprolactone were statically seeded with 1 x 10 cells/cm autologous bone marrow derived mononuclear cells. Eight TEVGs (7 seeded, 1 unseeded control) were implanted as inferior vena cava (IVC) interposition grafts in juvenile lambs. Subjects underwent bimonthly magnetic resonance angiography (Siemens 1.5 T) with vascular image analysis (www.BioimageSuite.org). One of 7-seeded grafts was explanted after 1 month and all others were explanted 6 months after implantation. Neotissue was characterized using qualitative histologic and immunohistochemical staining and quantitative biochemical analysis.

RESULTS

All grafts explanted at 6 months were patent and increased in volume as measured by difference in pixel summation in magnetic resonance angiography at 1 month and 6 months. The volume of seeded TEVGs at explant averaged 126.9% +/- 9.9% of their volume at 1 month. Magnetic resonance imaging demonstrated no evidence of aneurysmal dilation. TEVG resembled the native IVC histologically and had comparable collagen (157.9 +/- 26.4 microg/mg), elastin (186.9 +/- 16.7 microg/mg), and glycosaminoglycan (9.7 +/- 0.8 microg/mg) contents. Immunohistochemical staining and Western blot analysis showed that Ephrin-B4, a determinant of normal venous development, was acquired in the seeded grafts 6 months after implantation.

CONCLUSIONS

TEVGs demonstrate evidence of growth and venous development when implanted in the IVC of a juvenile lamb model.

EphrinB2 regulation by PTH and PTHrP revealed by molecular profiling in differentiating osteoblasts

With the aim of identifying new pathways and genes regulated by PTH(1-34) and PTH-related protein 1-141 [PTHrP(1-141)] in osteoblasts, this study was carried out using a mouse marrow stromal cell line, Kusa 4b10, that acquires features of the osteoblastic phenotype in long-term culture conditions. After the appearance of functional PTH receptor 1 (PTHR1) in Kusa 4b10 cells, they were treated with either PTH(1-34) or PTHrP(1-141), and RNA was subjected to Affymetrix whole mouse genome array. The microarray data were validated using quantitative real-time RT-PCR on independently prepared RNA samples from differentiated Kusa 4b10, UMR106 osteosarcoma cells, and primary mouse calvarial osteoblasts, as well as in vivo using RNA from metaphyseal bone after a single PTH injection to 3-wk-old and 6-mo-old ovariectomized rats. Of the 45,101 probes used on the microarray, 4675 were differentially expressed by >or=1.5 fold, with a false discovery rate <0.1. Among the regulated genes, ephrinB2 mRNA was upregulated in response to both PTH and PTHrP. This was confirmed by quantitative real-time PCR in vitro and in vivo. Increased ephrinB2 protein was also shown in vitro by Western blotting, and immunostaining of femur sections showed ephrinB2 in both osteoclasts and osteoblasts. Production of ephrinB2, as well as other ephrins or Eph family members, did not change during differentiation of Kusa 4b10 cells. Blockade of ephrinB2/EphB4 interaction resulted in inhibition of mineralization of Kusa 4b10 cells. Together with the shown effect of ephrinB2 promoting osteoblast differentiation and bone formation through action on EphB4, the data raise the possibility that PTH or PTHrP might regulate ephrinB2 to act in a paracrine or autocrine manner on EphB4 or EphB2 in the osteoblast, contributing as a local event to the anabolic action of PTH or PTHrP.

The anti-tumour agent, cisplatin, and its clinically ineffective isomer, transplatin, produce unique gene expression profiles in human cells

Cisplatin is a DNA-damaging anti-cancer agent that is widely used to treat a range of tumour types. Despite its clinical success, cisplatin treatment is still associated with a number of dose-limiting toxic side effects. The purpose of this study was to clarify the molecular events that are important in the anti-tumour activity of cisplatin, using gene expression profiling techniques. Currently, our incomplete understanding of this drug's mechanism of action hinders the development of more efficient and less harmful cisplatin-based chemotherapeutics. In this study the effect of cisplatin on gene expression in human foreskin fibroblasts has been investigated using human 19K oligonucleotide microarrays. In addition its clinically inactive isomer, transplatin, was also tested. Dualfluor microarray experiments comparing treated and untreated cells were performed in quadruplicate. Cisplatin treatment was shown to significantly up- or down-regulate a consistent subset of genes. Many of these genes responded similarly to treatment with transplatin, the therapeutically inactive isomer of cisplatin. However, a smaller proportion of these transcripts underwent differential expression changes in response to the two isomers. Some of these genes may constitute part of the DNA damage response induced by cisplatin that is critical for its anti-tumour activity. Ultimately, the identification of gene expression responses unique to clinically active compounds, like cisplatin, could thus greatly benefit the design and development of improved chemotherapeutics.

Control of blood vessel identity: from embryo to adult

Arteries and veins have been historically defined by the direction of blood flow and oxygen tension within the vessel, in addition to their functional, hemodynamic, and anatomical differences. It is now known that the molecular identity of these vessels is genetically predetermined, with specific molecular pathways activated during the development of arteries and veins. Eph-B4 is a determinant of venous differentiation and Ephrin-B2 is a determinant of arterial differentiation. Placement of a vein into the higher pressure and flow of the arterial circulation results in adaptation of the vein to the arterial environment. There is selective loss of Eph-B4 expression without induction of Ephrin-B2 expression during vein graft adaptation. These findings suggest that loss of venous identity is the crucial mechanism in vein graft adaptation and that developmentally critical determinants of vessel identity are plastic during adult life.

Expression and prognostic significance of EFNB2 and EphB4 genes in patients with oesophageal squamous cell carcinoma

OBJECTIVE

Tyrosine kinases and its receptors play important roles in growth, migration, and invasion of malignant cells. Among those, there are only few reports examining the expression pattern of Eph/ephrin signalling system in oesophageal carcinoma. The prognostic importance of ephrin-B2 ligand (EFNB2) and its receptor EphB4, and its correlation with clinicopathologic characteristics are yet to be delineated in patients with oesophageal carcinoma.

MATERIALS AND METHODS

EFNB2 gene and EphB4 receptor gene were examined of mRNA specimens in 61 patients with oesophageal squamous cell carcinoma using reverse-transcriptase polymerase chain reaction. EFNB2 protein was selectively examined using an immunohistochemical analysis.

RESULTS

EFNB2 mRNA expression was detected in 38 (62.3%) and EphB4 expression was found in 44 (72.1%) out of 61 cancer tissues analysed. There was a statistically significant correlation between EFNB2 expression and number of lymph node metastasis (P<0.05), and a trend toward statistical significance for correlation between EFNB2 expression and American Joint Committee on Cancer Classification Stage (P<0.1), indicating that EFNB2 expression was up-regulated by advancement of the disease process. EFNB2 protein was strongly expressed in tumour with high mRNA EFNB2 expression and was weakly expressed in tumour with low mRNA expression in some representative tumours. The 5-year overall survival rate (23%) of patients with positive EFNB2 gene expression was significantly worse than 55% of negative expression (P<0.05). The results of multivariate analysis of prognosticators for survival showed that positive EFNB2 gene expression (P<0.01) and number of lymph node metastasis (P<0.05) were identified as significant factors indicative of a poorer survival.

CONCLUSIONS

EFNB2 gene expression may be a biological marker and a useful prognostic indicator in patients with oesophageal squamous cell carcinoma.

EphB2 and ephrin-B1 expressed in the adult kidney regulate the cytoarchitecture of medullary tubule cells through Rho family GTPases

Eph receptors and ephrin ligands are membrane-bound cell-cell communication molecules with well-defined functions in development, but their expression patterns and functions in many adult tissues are still largely unknown. We have detected substantial levels of the EphB2 and EphB6 receptors and the ephrin-B1 ligand in the adult mouse kidney by RT-PCR amplification. Immunolocalization experiments revealed that EphB2 is localized in the tubules of the inner and outer medulla and EphB6 is in the tubules of the outer medulla and cortex. By contrast, ephrin-B1 was detected in tubules throughout the whole nephron. Consistent with the overlapping expression of the EphB2 receptor and the ephrin-B1 ligand in the medulla, EphB2 is tyrosine-phosphorylated, and therefore activated, in the kidney. In the outer medulla, however, EphB2 signaling may be attenuated by the co-expressed kinase-inactive EphB6 receptor. Interestingly, we found that EphB signaling induces RhoA activation and Rac1 inactivation as well as cell retraction, enlargement of focal adhesions and prominent stress fibers in primary cultures of medullary tubule cells. These results suggest that EphB receptor signaling through Rho family GTPases regulates the cytoarchitecture and spatial organization of the tubule cells in the adult kidney medulla and, therefore, may affect the reabsorption ability of the kidney.

Functional ephrin-B2 expression for promotive interaction between arterial and venous vessels in postnatal neovascularization

BACKGROUND

Ephrin-B2, one of the transmembrane ligands, is a genetic marker of arterial endothelial cells (ECs) at embryonic stages and is essential for cardiovascular development, but its roles in ischemic cardiovascular disease are not well understood. In this study, we focused on the function of ephrin-B2 in postnatal neovascularization.

METHODS AND RESULTS

We found that ephrin-B2 is exclusively expressed and significantly upregulated in the arterial vasculature after the initial angiogenic responses in tissue ischemia. Upregulation of ephrin-B2 is also observed in EC cordlike formation in vitro. Interestingly, ephrin-B2 expression on ECs was enhanced by promotive angiogenic growth factors, such as vascular endothelial growth factor, basic fibroblast growth factor, and hepatocyte growth factor, whereas it was attenuated by angiopoietin-1, a factor for blood vessel maturation. Moreover, an ephrin-B2-rich environment was shown to induce neovascularization mainly through venous angiogenesis in an in vivo cornea micropocket assay.

CONCLUSIONS

Our study indicates that the ephrin-B2 ligand is likely to have functional expression on angiogenic arterial ECs and induce a subsequent promotive effect on venous vessels during postnatal neovascularization.

EphB receptor-binding peptides identified by phage display enable design of an antagonist with ephrin-like affinity

The Eph receptor tyrosine kinases are overexpressed in many pathologic tissues and have therefore emerged as promising drug target candidates. However, there are few molecules available that can selectively bind to a single Eph receptor and not other members of this large receptor family. Here we report the identification by phage display of peptides that bind selectively to different receptors of the EphB class, including EphB1, EphB2, and EphB4. Peptides with the same EphB receptor specificity compete with each other for binding, suggesting that they have partially overlapping binding sites. In addition, several of the peptides contain amino acid motifs found in the G-H loop of the ephrin-B ligands, which is the region that mediates high-affinity interaction with the EphB receptors. Consistent with targeting the ephrin-binding site, the higher affinity peptides antagonize ephrin binding to the EphB receptors. We also designed an optimized EphB4-binding peptide with affinity comparable with that of the natural ligand, ephrin-B2. These peptides should be useful as selective inhibitors of the pathological activities of EphB receptors and as targeting agents for imaging probes and therapeutic drugs.

Tyr-298 in ephrinB1 is critical for an interaction with the Grb4 adaptor protein

The Eph family of receptor tyrosine kinases and their membrane-bound ligands, the ephrins, are thought to play a role in the regulation of cell adhesion and migration during development by mediating cell-to-cell signalling events. The transmembrane ephrinB protein is a bidirectional signalling molecule that sends a forward signal through the activation of its cognate receptor tyrosine kinase residing on another cell. The reverse signal is transduced into the ephrinB-expressing cell via tyrosine phosphorylation of its conserved C-terminal cytoplasmic domain. Previous work from our laboratory has implicated the activated FGFR1 (fibroblast growth factor receptor 1) as a regulator of a de-adhesion signal that results from overexpression of ephrinB1. In the present study, we report the isolation of Xenopus Grb4 (growth-factor-receptor-bound protein 4), an ephrinB1-interacting protein, and we show that when expressed in Xenopus oocytes, ephrinB1 interacts with Grb4 in the presence of an activated FGFR1. Amino acid substitutions were generated in Grb4, and the resulting mutants were expressed along with ephrinB1 and an activated FGFR in Xenopus oocytes. Co-immunoprecipitation analysis shows that the FLVR motif within the Src homology 2 domain of Xenopus Grb4 is vital for this phosphorylation-dependent interaction with ephrinB1. More importantly, using deletion and substitution analysis we identify the tyrosine residue at position 298 of ephrinB1 as being required for the physical interaction with Grb4, whereas Tyr-305 and Tyr-310 are dispensable. Moreover, we show that the region between amino acids 301 and 304 of ephrinB1 is also required for this critical tyrosine-phosphorylation-dependent event.

Human dendritic cells express neuronal Eph receptor tyrosine kinases: role of EphA2 in regulating adhesion to fibronectin

Eph receptor tyrosine kinases and their ligands, the ephrins, have been primarily described in the nervous system for their roles in axon guidance, development, and cell intermingling. Here we address whether Eph receptors may also regulate dendritic cell (DC) trafficking. Reverse transcription-polymerase chain reaction (RT-PCR) analysis showed that DCs derived from CD34+ progenitors, but not from monocytes, expressed several receptors, in particular EphA2, EphA4, EphA7, EphB1, and EphB3 mRNA. EphB3 was specifically expressed by Langerhans cells, and EphA2 and EphA7 were expressed by both Langerhans- and interstitial-type DCs. EphA and EphB protein expression on DCs generated in vitro was confirmed by staining with ephrin-A3-Fc and ephrin-B3-Fc fusion proteins that bind to different Eph members, in particular EphA2 and EphB3. Immunostaining with anti-EphA2 antibodies demonstrated the expression of EphA2 by immature DCs and by skin Langerhans cells isolated ex vivo. Interestingly, ephrin expression was detected in epidermal keratinocytes and also in DCs. Adhesion of CD34+-derived DCs to fibronectin, but not to poly-l-lysine, was increased in the presence of ephrin-A3-Fc, a ligand of EphA2, through a beta1 integrin activation pathway. As such, EphA2/ephrin-A3 interactions may play a role in the localization and network of Langerhans cells in the epithelium and in the regulation of their trafficking.

Change of gene expression profiles in the retina following optic nerve injury

The purpose of the present study was to search for changes in gene expression patterns in the retina following optic nerve injury. We conducted a subtractive hybridization for comparison of the mRNAs in those retinas receiving optic nerve crush injury and those receiving sham operation. Both forward and reverse subtractions were carried out for 8-h and 24-h time points postoperatively. Resultant subtractive cDNA for each group was re-amplified and cloned to a plasmid. After DNA sequencing, the identity of subtractive cDNA was analyzed by blasting sequences to the Nonredundant gene database, Unigene database, and dbest database at NCBI. Thirty-four known genes and 32 EST were found in the forward subtractions. Forty-two known genes and 46 EST were found in reverse subtractions. Identities of the rest could not been found in the databases. To verify the subtractive results, RT-PCR was performed to test expression patterns of eight known genes found in the above analysis. Among these eight genes, seven demonstrated a statistically significant difference between the crushed eyes and the control eyes by quantitative image analysis. Together, our data show that expression of fatso, ephrin B2, NonO, Zfx, vitronectin, and XLRS increased after optic nerve injury, and expression of stathmin exhibited reduction after injury.

A role of EphB4 receptor and its ligand, ephrin-B2, in erythropoiesis

Erythropoiesis is regulated not only by erythropoietin but also by microenvironments which are composed of transmembrane molecules. We have previously shown that a receptor tyrosine kinase EphB4 is predominantly expressed on human erythroid progenitors in bone marrow. EphB4 is expressed in approximately 45% of hematopoietic progenitor cells, which are CD34-positive and c-Kit-positive in human umbilical cord blood (hUCB). The transmembrane ligand for EphB4 or ephrin-B2 is expressed on bone marrow stromal cells and arterial endothelial cells. When such EphB4-positive hematopoietic progenitor cells were co-cultured with stromal cells which express ephrin-B2, they were immediately detached from stromal cells and differentiated to mature erythroid cells. At that time, expression of EphB4 immediately down-regulated. In contrast, on ephrin-B2 non-expressing stromal cells, they remained EphB4-positive cells and the generated number of mature erythroid cells was less than that on ephrin-B2 expressing stromal cells. Additionally, ephrin-B2 expression on endothelial cells up-regulated under hypoxic condition. Taken together, we propose that one of the molecular cues that regulate erythropoiesis is ephrin-B2 on stromal cells.

Origin, differentiation, and maturation of human pulmonary veins

Recent studies on human embryonic and fetal lungs show that the pulmonary arteries form by vasculogenesis. Little is known of the early development of the pulmonary veins. Using immunohistochemical techniques and serial reconstruction, we studied 18 fetal and neonatal lungs. Sections were stained with antibodies specific for endothelium (CD31, von Willebrand factor) and smooth muscle (alpha and gamma smooth muscle actin, smooth muscle myosin, calponin, caldesmon, and desmin) and antibodies specific for the matrix glycoprotein tenascin, the receptor protein tyrosine kinase EphB4, and its ligand ephrinB2. Kiel University-raised antibody number 67 (Ki67) expression allowed qualitative assessment of cell replication. By 34 d gestation, there was continuity between the aortic sac, pulmonary arteries, capillaries, pulmonary veins, and atrium. The pulmonary veins formed by vasculogenesis in the mesenchyme surrounding the terminal buds during the pseudoglandular period and probably by angiogenesis in the canalicular and alveolar stages. EphB4 and ephrinB2 did not distinguish between presumptive venous and arterial endothelium as they do in mouse. All venous smooth muscle cells derived directly from the mesenchyme, gradually acquiring smooth muscle specific proteins from 56 d gestation. Thus, both pulmonary arteries and veins arise by vasculogenesis, but the origins of their smooth muscle cells and their cytoskeletal protein content are different.

Activation of EphB2 and its ligands promotes vascular smooth muscle cell proliferation

EphB2 and its ligands regulate interactions between endothelial and mesenchymal cells in developing arteries. In adult arteries, the relationship between smooth muscle cells and overlying intact endothelium is responsible for maintaining the health of the vessel. Heparin inhibits vascular smooth muscle cell growth in culture and intimal hyperplasia following endothelial denudation. Using gene microarrays, we identified the tyrosine kinase receptor EphB2 as being differentially expressed in response to continuous intravenous heparin administration in the rabbit model of arterial injury. EphB2 protein levels increased in cultured bovine vascular smooth muscle cells following serum stimulation and were decreased in a dose-dependent fashion by heparin. Fc chimeras of the binding domain of the EphB2 ligands blocked the formation of the EphB2 ligand-receptor complex and reduced growth of serum-stimulated vascular smooth muscle cells in a dose-dependent fashion. Activation of the ligand by an Fc chimera to EphB2 followed a parabolic dose-response growth curve, indicating growth stimulation until the chimera begins to compete with native receptors. Co-administration of EphB2/Fc chimera with heparin shifted the dose-response curve to the right. These data indicate a possible new route of Heparin's antiproliferative effect and a role of EphB2 and its ligands in vascular smooth muscle cell proliferation.

Arterial identity of endothelial cells is controlled by local cues

The ephrins and their Eph receptors comprise the largest family of receptor tyrosine kinases. Studies on mice have revealed an important function of ephrin-B2 and Eph-B4 for the development of the arterial and venous vasculature, respectively, but the mechanisms regulating their expression have not been studied yet. We have cloned a chick ephrin-B2 cDNA probe. Expression was observed in endothelial cells of extra- and intraembryonic arteries and arterioles in all embryos studied from day 2 (stage 10 HH, before perfusion of the vessels) to day 16. Additionally, expression was found in the somites and neural tube in early stages, and later also in the smooth muscle cells of the aorta, parts of the Müllerian duct, dosal neural tube, and joints of the limbs. We isolated endothelial cells from the internal carotid artery and the vena cava of 14-day-old quail embryos and grafted them separately into day-3 chick embryos. Reincubation was performed until day 6 and the quail endothelial cells were identified with the QH1 antibody. The grafted arterial and venous endothelial cells expressed ephrin-B2 when they integrated into the lining of arteries. Cells that were not integrated into vessels, or into vessels other than arteries, were ephrin-B2-negative. The studies show that the expression of the arterial marker ephrin-B2 is controlled by local cues in arterial vessels of older embryos. Physical forces or the media smooth muscle cells may be involved in this process.

Development of the visual system of the chick. II. Mechanisms of axonal guidance

The quest to understand axonal guidance mechanisms requires exact and multidisciplinary analyses of axon navigation. This review is the second part of an attempt to synthesise experimental data with theoretical models of the development of the topographic connection of the chick retina with the tectum. The first part included classic ideas from developmental biology and recent achievements on the molecular level in understanding cytodifferentiation and histogenesis [J. Mey, S. Thanos, Development of the visual system of the chick. (I) Cell differentiation and histogenesis, Brain Res. Rev. 32 (2000) 343-379]. The present part deals with the question of how millions of fibres exit from the eye, traverse over several millimetres and spread over the optic tectum to assemble a topographic map, whose precision accounts for the sensory performance of the visual system. The following topics gained special attention in this review. (i) A remarkable conceptual continuity between classic embryology and recent molecular biology has revealed that positional cellular specification precedes and determines the formation of the retinotectal map. (ii) Graded expression of asymmetric genes, transcriptional factors and receptors for signal transduction during early development seem to play a crucial role in determining the spatial identity of neurons within surface areas of retina and optic tectum. (iii) The chemoaffinity hypothesis constitutes the conceptual framework for development of the retinotopic organisation of the primary visual pathway. Studies of repulsive factors in vitro developed the original hypothesis from a theoretical postulate of chemoattraction to an empirically supported concept based on chemorepulsion. (iv) The independent but synchronous development of retina and optic tectum in topo-chronologically corresponding patterns ensures that ingrowing retinal axons encounter receptive target tissue at appropriate locations, and at the time when connections are due to be formed. (v) The growth cones of the retino-fugal axons seem to be guided both by local cues on glial endfeet and within the extracellular matrix. On the molecular level, the ephrins and their receptors have emerged as the most likely candidates for the material substrate of a topographic projection along the anterior-posterior axis of the optic tectum. Yet, since a number of alternative molecules have been proposed for the same function, it remains the challenge for the near future to define the proportional contribution of each one of the individual mechanisms proposed by matching theoretical predictions with the experimental evidence.

Ephrin-B reverse signaling is mediated by a novel PDZ-RGS protein and selectively inhibits G protein-coupled chemoattraction

Transmembrane B ephrins and their Eph receptors signal bidirectionally. However, neither the cell biological effects nor signal transduction mechanisms of the reverse signal are well understood. We describe a cytoplasmic protein, PDZ-RGS3, which binds B ephrins through a PDZ domain, and has a regulator of heterotrimeric G protein signaling (RGS) domain. PDZ-RGS3 can mediate signaling from the ephrin-B cytoplasmic tail. SDF-1, a chemokine with a G protein-coupled receptor, or BDNF, act as chemoattractants for cerebellar granule cells, with SDF-1 action being selectively inhibited by soluble EphB receptor. This study reveals a pathway that links reverse signaling to cellular guidance, uncovers a novel mode of control for G proteins, and demonstrates a mechanism for selective regulation of responsiveness to neuronal guidance cues.

Comparative analysis of the gene-dense ACHE/TFR2 region on human chromosome 7q22 with the orthologous region on mouse chromosome 5

Chromosome 7q22 has been the focus of many cytogenetic and molecular studies aimed at delineating regions commonly deleted in myeloid leukemias and myelodysplastic syndromes. We have compared a gene-dense, GC-rich sub-region of 7q22 with the orthologous region on mouse chromosome 5. A physical map of 640 kb of genomic DNA from mouse chromosome 5 was derived from a series of overlapping bacterial artificial chromosomes. A 296 kb segment from the physical map, spanning ACHE: to Tfr2, was compared with 267 kb of human sequence. We identified a conserved linkage of 12 genes including an open reading frame flanked by ACHE: and Asr2, a novel cation-chloride cotransporter interacting protein Cip1, Ephb4, Zan and Perq1. While some of these genes have been previously described, in each case we present new data derived from our comparative sequence analysis. Adjacent unfinished sequence data from the mouse contains an orthologous block of 10 additional genes including three novel cDNA sequences that we subsequently mapped to human 7q22. Methods for displaying comparative genomic information, including unfinished sequence data, are becoming increasingly important. We supplement our printed comparative analysis with a new, Web-based program called Laj (local alignments with java). Laj provides interactive access to archived pairwise sequence alignments via the WWW. It displays synchronized views of a dot-plot, a percent identity plot, a nucleotide-level local alignment and a variety of relevant annotations. Our mouse-human comparison can be viewed at http://web.uvic.ca/~bioweb/laj.html. Laj is available at http://bio.cse.psu.edu/, along with online documentation and additional examples of annotated genomic regions.

Expression of ephrinB2 identifies a stable genetic difference between arterial and venous vascular smooth muscle as well as endothelial cells, and marks subsets of microvessels at sites of adult neovascularization

The transmembrane ligand ephrinB2 and its receptor tyrosine kinase EphB4 are molecular markers of embryonic arterial and venous endothelial cells, respectively, and are essential for angiogenesis. Here we show that expression of ephrinB2 persists in adult arteries where it extends into some of the smallest diameter microvessels, challenging the classical view that capillaries have neither arterial nor venous identity. EphrinB2 also identifies arterial microvessels in several settings of adult neovascularization, including tumor angiogenesis, contravening the dogma that tumor vessels arise exclusively from postcapillary venules. Unexpectedly, expression of ephrinB2 also defines a stable genetic difference between arterial and venous vascular smooth muscle cells. These observations argue for revisions of classical concepts of capillary identity and the topography of neovascularization. They also imply that ephrinB2 may be functionally important in neovascularization and in arterial smooth muscle, as well as in embryonic angiogenesis.

Complementary expression of transmembrane ephrins and their receptors in the mouse spinal cord: a possible role in constraining the orientation of longitudinally projecting axons

In the developing spinal cord, axons project in both the transverse plane, perpendicular to the floor plate, and in the longitudinal plane, parallel to the floor plate. For many axons, the floor plate is a source of long- and short-range guidance cues that govern growth along both dimensions. We show here that B-class transmembrane ephrins and their receptors are reciprocally expressed on floor plate cells and longitudinally projecting axons in the mouse spinal cord. During the period of commissural axon pathfinding, B-class ephrin protein is expressed at the lateral floor plate boundaries, at the interface between the floor plate and the ventral funiculus. In contrast, B-class Eph receptors are expressed on decussated commissural axon segments projecting within the ventral funiculus, and on ipsilaterally projecting axons constituting the lateral funiculus. Soluble forms of all three B-class ephrins bind to, and induce the collapse of, commissural growth cones in vitro. The collapse-inducing activity associated with B-class ephrins is likely to be mediated by EphB1. Taken together, these data support a possible role for repulsive B-class Eph receptor/ligand interactions in constraining the orientation of longitudinal axon projections at the ventral midline.

The C1orf9 gene encodes a putative transmembrane member of a novel protein family

Here we report the characterization of a human mRNA encoding a novel protein denoted C1orf9 (chromosome 1 open reading frame 9). The cDNA sequence, derived from a testis cDNA library, contains 5700 bp which encodes an open reading frame of 1254 amino acids. The deduced protein contains a putative N-terminal signal peptide and one putative transmembrane region, indicating membrane localization. No significant homology was found with known characterized proteins. However, a 150 amino acid region has significant homology to deduced protein sequences from other organisms, including Caenorhabditis elegans (43% identity), Saccharomyces cerevisiae (47% identity), Schizosaccharomyces pombe (48% identity), and two proteins from Arabidopsis thaliana (42% and 40% identity), suggesting a novel family of conserved domains. The C1orf9 gene was assigned to chromosome 1q24. The gene spans approximately 78.7 kb and is organized into at least 24 exons. Expression analysis revealed a single C1orf9 mRNA species of approximately 6.0 kb with a predominant expression in pancreas and testis, and only low levels of expression in other tissues examined.

Ephrin-B2 is a candidate ligand for the Eph receptor, EphB6

No ligand has hitherto been designated for the Eph receptor tyrosine kinase family member, EphB6. Here, expression of an EphB6 ligand in the pro-B leukemic cell line, Reh, is demonstrated by binding of soluble EphB6-Fc fusion protein to the Reh cells. The ligand belongs to the subgroup of membrane spanning ligands, as suggested by the fact that phosphatidylinositol-specific phospholipase C treatment did not abrogate binding of EphB6-Fc. Two transmembrane Eph receptor ligands, ephrin-B1 and ephrin-B2, were identified in Reh cells. Analysis of EphB6-Fc fusion protein binding to ephrin-B1 or ephrin-B2 transfected COS cells revealed a high-affinity saturable binding between EphB6-Fc and ephrin-B2, but not with ephrin-B1. In mice, EphB6 has previously been shown to be expressed in thymus. Here, we show expression of EphB6 in human thymus, as well as the expression of ephrin-B2 in both human and mouse thymus. We conclude that ephrin-B2 may be a physiological ligand for the EphB6 receptor.

A splice variant of human ephrin-A4 encodes a soluble molecule that is secreted by activated human B lymphocytes

Ephrin-A4 is a ligand for the erythropoietin-producing hepatocellular (Eph) receptor family of tyrosine kinases. We have identified a secreted form of ephrin-A4, denoted ephrin-A4 (s), which is encoded by an alternatively spliced mRNA and is produced by in vivo activated B cells in tonsils. Blood B cells secrete ephrin-A4 (s) upon stimulation via the B-cell antigen receptor. A subpopulation of tonsil cells in the crypts with a dendritic cell phenotype was shown to express EphA2, an Eph receptor tyrosine kinase that was found to be capable of binding an ephrin-A4 immunoglobulin chimeric protein. We conclude that ephrin-A4 (s) may play a role in the interaction between activated B lymphocytes and dendritic cells in human tonsils. (Blood. 2000;95:221-230)

A splice variant of human ephrin-A4 encodes a soluble molecule that is secreted by activated human B lymphocytes

Ephrin-A4 is a ligand for the erythropoietin-producing hepatocellular (Eph) receptor family of tyrosine kinases. We have identified a secreted form of ephrin-A4, denoted ephrin-A4 (s), which is encoded by an alternatively spliced mRNA and is produced by in vivo activated B cells in tonsils. Blood B cells secrete ephrin-A4 (s) upon stimulation via the B-cell antigen receptor. A subpopulation of tonsil cells in the crypts with a dendritic cell phenotype was shown to express EphA2, an Eph receptor tyrosine kinase that was found to be capable of binding an ephrin-A4 immunoglobulin chimeric protein. We conclude that ephrin-A4 (s) may play a role in the interaction between activated B lymphocytes and dendritic cells in human tonsils. (Blood. 2000;95:221-230)

Comparative analysis of embryonic gene expression defines potential interaction sites for Xenopus EphB4 receptors with ephrin-B ligands

The Eph family of receptor tyrosine kinases and their ligands, the ephrins, act as signaling molecules regulating the migratory behavior of neurons and neural crest cells, and are implicated in tissue patterning, blood vessel formation, and tumorigenesis. On the basis of structural similarities and overlapping binding specificities, Eph receptors as well as their ligands can be divided into A and B subfamilies with orthologues found in all vertebrates. We describe here the isolation of cDNAs encoding Xenopus EphB4 receptors and show that embryonic expression is prominently associated with the developing vasculature, newly forming somites, the visceral arches, and non-neuronal tissues of the embryonic head. In a screen to identify potential ligands for EphB4 in Xenopus embryos, we isolated cDNAs for the Xenopus ephrin-B2 and -B3, which demonstrates that the Xenopus genome harbors genes encoding orthologues to all three currently known mammalian ephrin-B genes. We next performed in situ hybridizations to identify tissues and organs where EphB4 receptors may encounter ephrin-B ligands during embryonic development. Our analysis revealed distinct, but overlapping patterns of ephrin-B gene expression. Interestingly, each ephrin-B ligand displayed expression domains either adjacent to or within EphB4-expressing tissues. These findings indicate that EphB4 receptors may interact in vivo with multiple B-class ephrins. The expression patterns also suggest that EphB4 receptors and their ligands may be involved in visceral arch formation, somitogenesis, and blood vessel development.

Expression of Eph receptor tyrosine kinases and their ligands in chick embryonic motor neurons and hindlimb muscles

Evidence is accumulating that Eph receptor tyrosine kinases and their ligands regulate cell migration and axonal guidance during development. It was previously found that one of the Eph receptors, EphA4, is transiently expressed in subsets of chick embryonic motor neurons. Here, the expression of EphA and ephrin-A subfamily members was further examined, and the dynamic patterns of expression in chick embryonic motor neurons found. EphA3, EphA4, ephrin-A2, and ephrin-A5 were also expressed in the connective tissues of limb muscles and EphA3 and EphA4 expressing motor neurons innervated EphA3 and EphA4 expressing limb muscles, respectively. These spatiotemporal expression patterns suggest that EphA and ephrin-A proteins play important roles in muscle patterning and motor axonal guidance.

Identification and characterization of splice variants of ephrin-A3 and ephrin-A5

Ephrins and Eph receptors have been implicated to play important roles in axon guidance. A variable spacer region exists that differs significantly among distinct ephrins. An ephrin-A5 isoform has previously been isolated which lacks 27 amino acids within the spacer region. The expression and biological activities of this isoform, as well as the existence of isoforms for other ephrins that show variation within the spacer region, remain unknown. We report here a novel alternatively spliced isoform of ephrin-A3 which lacks the corresponding variable region. When compared to the longer isoforms, the shorter isoforms of both ephrin-A3 and ephrin-A5 remained less prominent in the brain during development, though their expression increased at postnatal stages. In addition, they could inhibit neurite outgrowth of dorsal root ganglia (DRG) neurons, suggesting that the corresponding variable regions were not essential for their axon guidance activities.

Symmetrical mutant phenotypes of the receptor EphB4 and its specific transmembrane ligand ephrin-B2 in cardiovascular development

Ephrin-B2 is a transmembrane ligand that is specifically expressed on arteries but not veins and that is essential for cardiovascular development. However, ephrin-B2 is also expressed in nonvascular tissues and interacts with multiple EphB class receptors expressed in both endothelial and nonendothelial cell types. Thus, the identity of the relevant receptor for ephrin-B2 and the site(s) where these molecules interact to control angiogenesis were not clear. Here we show that EphB4, a specific receptor for ephrin-B2, is exclusively expressed by vascular endothelial cells in embryos and is preferentially expressed on veins. A targeted mutation in EphB4 essentially phenocopies the mutation in ephrin-B2. These data indicate that ephrin-B2-EphB4 interactions are intrinsically required in vascular endothelial cells and are consistent with the idea that they mediate bidirectional signaling essential for angiogenesis.

Expression of receptor tyrosine kinase HTK (hepatoma transmembrane kinase) and HTK ligand by human leukemia-lymphoma cell lines

HTK (hepatoma transmembrane kinase) is a receptor tyrosine kinase belonging to the EPH subfamily of tyrosine kinases. Binding of its ligand (HTKL) results in tyrosine phosphorylation of HTK. In the present study, we analyzed the possible involvement of this ligand-receptor signalling system in hematopoiesis by examining the expression of both HTK and HTKL in a large and comprehensive panel of 70 continuous human leukemia-lymphoma cell lines. HTK and HTKL mRNA expression were analyzed by reverse transcriptase-polymerase chain reaction (RT-PCR). HTK mRNA was detected in 68/70 cell lines; 58/70 cell lines were positive for HTKL mRNA expression; consequently, co-expression of both receptor and ligand was demonstrated in the majority of cell lines. Collectively, the wide-spread expression suggests a role for this ligand-receptor pair in hematopoietic development and/or function. Investigation of the details of signal transduction pathway that is activated by the HTK tyrosine kinase will help to define the exact biological function of the HTK-HTKL system.

The carboxyl terminus of B class ephrins constitutes a PDZ domain binding motif

Ephrin B proteins function as ligands for B class Eph receptor tyrosine kinases and are postulated to possess an intrinsic signaling function. The sequence at the carboxyl terminus of B-type ephrins contains a putative PDZ binding site, providing a possible mechanism through which transmembrane ephrins might interact with cytoplasmic proteins. To test this notion, a day 10.5 mouse embryonic expression library was screened with a biotinylated peptide corresponding to the carboxyl terminus of ephrin B3. Three of the positive cDNAs encoded polypeptides with multiple PDZ domains, representing fragments of the molecule GRIP, the protein syntenin, and PHIP, a novel PDZ domain-containing protein related to Caenorhabditis elegans PAR-3. In addition, the binding specificities of PDZ domains previously predicted by an oriented library approach (Songyang, Z., Fanning, A. S., Fu, C., Xu, J., Marfatia, S. M., Chishti, A. H., Crompton, A., Chan, A. C., Anderson, J. M., and Cantley, L. C. (1997) Science 275, 73-77) identified the tyrosine phosphatase FAP-1 as a potential binding partner for B ephrins. In vitro studies demonstrated that the fifth PDZ domain of FAP-1 and full-length syntenin bound ephrin B1 via the carboxyl-terminal motif. Lastly, syntenin and ephrin B1 could be co-immunoprecipitated from transfected COS-1 cells, suggesting that PDZ domain binding of B ephrins can occur in cells. These results indicate that the carboxyl-terminal motif of B ephrins provides a binding site for specific PDZ domain-containing proteins, which might localize the transmembrane ligands for interactions with Eph receptors or participate in signaling within ephrin B-expressing cells.

Segregation of the embryonic vascular and hemopoietic systems

The origin of endothelial cells and their subsequent assembly into the primary vascular system have been mostly analyzed in the avian embryo. Following the discovery of specific growth factors and their cognate receptors, the molecular mechanisms underlying these processes have been unraveled in both birds and mammals. In particular, experimental studies of the angiogenic vascular endothelial growth factor (VEGF) and its receptors, carried out in both vertebrate classes, have provided significant insight into the developmental biology of endothelial cells. The VEGF receptor VEGFR2 is the earliest marker known to be expressed by endothelial precursor cells of avian and mouse embryos. Based on the localization of VEGFR2+ cells in the avian embryo and on clonal culture experiments, two types of endothelial precursor cells can be distinguished from gastrulation stages onward: posterior mesodermal VEGFR2+ hemangioblasts, which have the capacity to differentiate into endothelial and hemopoietic cells, and anterior VEGFR2+ angioblasts, which can only give rise to endothelial cells.Key words: hemangioblast, endothelial cell, hemopoietic cell, embryo.

Cell-type specific and estrogen dependent expression of the receptor tyrosine kinase EphB4 and its ligand ephrin-B2 during mammary gland morphogenesis

Morphogenesis of the mammary gland occurs mainly during adult life and is dependent on a complex interplay of hormonal, cell-cell and cell-matrix interactions. The molecular mechanisms involved in pattern formation of the mammary epithelium in adult life are poorly understood. Recently, several members of the Eph family of receptor tyrosine kinases and their ligands have been shown to participate in pattern formation during embryogenesis and conceivably may fulfill similar functions during adult morphogenesis. We have investigated the expression of a member of this family, EphB4, and its cognate ligand, ephrin-B2, during normal and malignant mouse mammary morphogenesis. A spatially, temporarily and hormonally coordinated expression of both the receptor and ligand was observed. The receptor was predominantly localized in the myoepithelial cells surrounding the ducts and alveoli whereas ligand expression was limited to the luminal epithelial cells. Expression of both was induced at the onset of gland morphogenesis at puberty and was differentially regulated during the estrus cycle. Ovariectomy of pre-pubertal or adult females abolished the expression of both receptor and ligand and administration of estrogen alone was sufficient to restore their normal expression. Disruption of the balanced expression was observed during experimental mouse mammary carcinogenesis. Ligand expression was lost at the onset of tumorigenesis and receptor expression shifted from myoepithelial to epithelial cells with progressive malignancy. These results implicate both the EphB4 receptor and its ligand ephrin-B2 in the hormone dependent morphogenesis of the mammary gland. Furthermore, their deregulated expression may contribute to mammary carcinogenesis.

Molecular distinction and angiogenic interaction between embryonic arteries and veins revealed by ephrin-B2 and its receptor Eph-B4

The vertebrate circulatory system is composed of arteries and veins. The functional and pathological differences between these vessels have been assumed to reflect physiological differences such as oxygenation and blood pressure. Here we show that ephrin-B2, an Eph family transmembrane ligand, marks arterial but not venous endothelial cells from the onset of angiogenesis. Conversely, Eph-B4, a receptor for ephrin-B2, marks veins but not arteries. ephrin-B2 knockout mice display defects in angiogenesis by both arteries and veins in the capillary networks of the head and yolk sac as well as in myocardial trabeculation. These results provide evidence that differences between arteries and veins are in part genetically determined and suggest that reciprocal signaling between these two types of vessels is crucial for morphogenesis of the capillary beds.

The ephrins and Eph receptors in neural development

The Eph receptors are the largest known family of receptor tyrosine kinases. Initially all of them were identified as orphan receptors without known ligands, and their specific functions were not well understood. During the past few years, a corresponding family of ligands has been identified, called the ephrins, and specific functions have now been identified in neural development. The ephrins and Eph receptors are implicated as positional labels that may guide the development of neural topographic maps. They have also been implicated in pathway selection by axons, the guidance of cell migration, and the establishment of regional pattern in the nervous system. The ligands are anchored to cell surfaces, and most of the functions so far identified can be interpreted as precise guidance of cell or axon movement. This large family of ligands and receptors may make a major contribution to the accurate spatial patterning of connections and cell position in the nervous system.

The Eph family receptors and ligands

The Eph family is the largest of all known tyrosine kinase receptor-ligand systems. They are expressed in distinct, but overlapping, spatial and temporal patterns during embryonic development and postnatal life, and function in a variety of morphogenic events. The best known function is their role in the guidance of migration of axons and cells in the nervous system through repulsive interactions. They may also play a role in angiogenesis, tissue patterning, and tumor formation.

Characterization of the genes for mouse LERK-3/Ephrin-A3 (Epl3), mouse LERK-4/Ephrin-A4 (Epl4), and human LERK-6/Ephrin-A2 (EPLG6): conservation of intron/exon structure

We have isolated the genes for the eph receptor family ligands mouse LERK-3/Ephrin-A3 (Epl3), mouse LERK-4/Ephrin-A4 (Epl4), and human LERK-6/Ephrin-A2 (EPLG6). These genes show a high level of conservation in their intron/exon structures encoding the receptor-binding region. In addition, the nucleotide sequences of the genes reveal the predicted cDNA sequence of mouse LERK-3/Ephrin-A3, mouse LERK-4/Ephrin-A4, and human LERK-6/Ephrin-A2.